The output capability of electrical transducers can be modified and expanded by selectively stimulating joined transducer segments. For example, piezoelectric actuators, an illustrative class of electrical transducers, can be constructed of stacked segments to provide a three-dimensional mechanical output. By combining cyclically alternating traction strokes of two or more piezoelectric actuators, "walking" motion can be produced.
At the start of tractional contact in conventional resonant actuators, such as resonant ultrasonic traveling wave motors, relative velocity between the actuator and a movable object results in initial sliding until firm contact is established. At the end of the traction portion of the cycle, sliding occurs again in preparation for the lift-off and retrace portions of the cycle. In such a system efficiency of electromechanical power conversion is reduced by sliding friction, particularly on retrace when the relative velocity is greatest. The electromechanical conversion efficiency of an actuating device with a conventional resonant drive is generally less than about 40%, with most of the loss caused by sliding friction. Rotors of resonant traveling wave piezoelectric motors are known to become uncomfortably hot to the touch after a few minutes of operation. Furthermore, resonant actuators that use circular or elliptical traction paths impart a time-varying normal force and a time-varying tangential (traction) force to the positioned object or rotor. As a result, linear motion is jerky and motors have torque ripple.
The speed of action of resonant actuators may be adjusted by altering the magnitude of the voltage swings or the mechanical strokes. However, resonant devices operate most efficiently at full power and full speed. Speed reduction by reducing amplitude also reduces efficiency because the energy allocated to sliding friction becomes an increasing fraction of the total available energy. As the size of the circular or elliptical motion is decreased, rubbing during retrace is exacerbated. The lower limit of speed is determined by the resonant amplitude at which traction cycling becomes intermittent or ceases altogether. Holding a controlled stationary position and applying a static traction force are beyond the operating scope of a resonant actuator.
Resonantly excited piezoelectric actuators of the prior art employ the thickness or extension piezoelectric deformation mode, wherein ferroelectric polarization is parallel to the applied electric field. Such polarization is degraded or even reversed if the applied potential causes an electric field anti-parallel to the direction of polarization. Resonance requires that large voltage swings be substantially symmetric about a floating median potential value. Elevation of the floating median value by at least half the peak-to-peak voltage swing above electrical ground avoids depolarization. This requires that the electrical circuitry operate at a relatively high voltage above ground, resulting in a safety hazard in large actuators.
Slowly varying direct current electrical sources, such as programmable DC power supplies, for example, have been used to control piezoelectric positioners. These power sources emulate class A amplifiers but have a restricted frequency response. All the reactive capacitive current flows through the amplifier output devices. A class A amplifier dissipates all of the available power internally under null excitation. The variable DC voltage is essentially free of superimposed high frequency ripple, and it provides smooth control and piezoelectric positioning at slow speeds (including zero speed) with relatively good positioning accuracy. At modest frequencies and voltages, programmable DC power supplies operate piezoelectric actuators as smooth walkers without losses from sliding friction. However, high efficiency is beyond the capability of a programmable DC power supply emulating a class A or class B amplifier. Operation becomes more difficult and less efficient above a few traction cycles per second or with voltages above about 200 volts. Furthermore, none of the known class A or class B linear amplifiers remain stable when driving an entirely capacitive load. Therefore, they ar not applicable to electromechanically efficient piezoelectric walking actuators except at the lowest portion of the amplifier's frequency band.
An electronic drive described in U.S. Pat. No. 4,628,275 issued to Skipper, et al. emulates a class A amplifier. The amplifier provides the high bipolar voltage swings necessary to operate piezoelectric shear actuators. However, the ultrasonic charge transfer cycles of the amplifier, even when holding a steady voltage, accelerate the rate of wear and fatigue in all mechanisms connecting the actuator to positioned objects. Furthermore, the amplifier requires high voltage DC power supplies, large and heavy transformers, and very fast switching devices to achieve modest electrical efficiencies. The use of AC-to-DC power converters and the presence of large reactive currents in output switching devices preclude efficiencies above about 80%.
Piezoelectric actuators capable of smooth walking are inherently well suited to operation in a vacuum, such as in orbiting space stations, because lubrication is not required. The high electromechanical efficiency of piezoelectric actuators, which exceeds 98% neglecting internal electrical losses, also precludes excessive heating during operation, thus eliminating the need for ancillary cooling that reduces overall system efficiency. Furthermore, piezoelectric actuators require no additional energy from the power source to maintain a stationary force.
Operation in a vacuum imposes similar thermal management requirements on the drive electronics. The need for heat removal decreases dramatically with increasing efficiency. Internal heat generation by an ideal electrical power source is negligible when piezoelectric actuators apply a stationary force or operate at low velocities. Ideally, the energy supplied by the drive system should equal the energy converted to useful mechanical work by the actuators. Thus, there is a need for a highly efficient electrical system to supply appropriate resonant waveforms for nonsinusoidal transducer output such as smooth walking motion by piezoelectric actuators.